Visualization of tubular objects is important for many applications, such as virtual colonoscopy (VC). Volume data is often rendered as a surface, as it is a good way to visualize complex objects. In order to visualize a solid object as a surface, it is necessary to first convert the input data into a mesh representation. In 1999, Bernardini et al. described one such approach for representation of 3D data using triangular mesh. In most cases, the objective is to visualize the exterior surface of the triangle mesh. However, in applications such as VC, it is more beneficial to visualize the interior surface.
In a virtual colonoscopy the patient undergoes a Computed Tomography (CT) scan, and the data collected, slices of 2D images, from the scan are used to create a 3D representation of the colon. VC aims to quantify the internal texture of the colon. Common VC visualization techniques include virtual fly-through (FT) such as taught in U.S. Pat. No. 6,331,116 and virtual fly-over (FO) such as taught in U.S. Published Application 2008/0069419, both incorporated herein by reference. Both techniques simulate a real colonoscopy by moving a virtual camera with a specific field of view along a planned path inside the colon, rendering internal views. Navigation can move from either the colon rectum (antegrade) or its cecum (retrograde).
Both FT and FO use the centerline as their navigation path; as the centerline is the locus of points distant from the colon boundary and provides complete visualization without blind areas. However, up to 20% of colonic mucosa is missed by a unidirectional fly-through, and about 8% are missed in fly-over, especially in the cases of poorly distended colon surfaces.
Thus, thorough/detailed VC examination, using FT, requires supine and prone antegrade and retrograde fly-throughs: a total of four fly-through operations, which is time consuming for medical staff. Fly-over does not suffer such limitations, yet it is computationally more complicated and requires two virtual cameras instead of one.
Other visualization techniques known in the art are not dependent on the centerline for navigation, but likewise require a planned path. Their visibility coverage exceeds that of traditional centerline-based fly-throughs. Still, polyps hidden between narrow haustral folds can be overlooked, and the resulting colon representation distorts the lumen, which distorts polyps and affects their visibility.
In 2006, a fly-over (FO) visualization technique was proposed by Hassouna eta/that overcame the limitations of existing approaches (especially fly-through and flattening). Splitting the entire colon into two halves, it gave each half a virtual camera for fly-over navigation. This technique covered up to 20% more than fly-through navigation. Additionally, only two traversals of the colon were required as compared to the four transversals for fly-throughs. However, splitting the colon requires much computation, and polyps located exactly at the split can potentially be missed. Splitting the colon could also result in ambiguities of the visualization of the colon surface, as the colon's toplogy may lead to allocating incorrect regions of the lumen at a particular visualization instance, if such regions are closer to the centerline than the correct location, leading to “holes” in the colon surface.
Thus, a method is needed that will provide more complete images, reduce computational time, and reduce memory requirements compared to technology of the prior art, while preserving VCs benefits for patients and practitioners.